In optical communication and so forth, among light emitted from a semiconductor laser, the light reflected from an optical fiber or the like returns to the semiconductor laser again. This light having returned upon reflection (reflection return light) may cause mode hopping of the semiconductor laser to come greatly causative of a noise which makes unstable the light lased from the laser. Accordingly, in order to control such reflection return light, an optical isolator is used which utilizes the Faraday effect.
Basic constitution of the optical isolator is shown in FIG. 1. Stated specifically, as shown in FIGS. 1(A) and (B), this optical isolator is mainly constituted of a first polarizer 1 which is disposed along the optical axis and the plane of polarization of which is in non-phase-shift in the plane of polarization of incident light, a Faraday rotator 2 which is disposed on the rear of the light emergent side of this first polarizer 1 and rotates the plane of polarization of incident light by 45 degrees, and a second polarizer 3 which is disposed on the rear of the light emergent side of this Faraday rotator 2 and has the plane of polarization standing inclined by 45 degrees in respect to the first polarizer 1.
The Faraday rotator 2, the first polarizer 1 and the second polarizer 3 are joined with an optical-purpose adhesive at their interfaces to make up an optical isolator element, and also this optical isolator element, and permanent magnets 4 which bring into saturation magnetization the Faraday rotator 2 of this optical isolator element, are fastened by bonding to a holder 5 via the optical-purpose adhesive to make up an optical isolator.
Now, as conventional optical-purpose adhesives, those making use of an epoxy resin as a primary agent and an amine type compound as a curing agent are known in the art.
However, the conventional optical-purpose adhesives of this kind have a disadvantage that cured products tend to cause discoloration and deterioration which are due to temperature and humidity after curing. There have been problems such that, in a thermo-hygrostat test at 85° C./85% RH for 2,000 hours which is conducted using the optical isolator, the optical isolator element and the permanent magnets may come separate from the holder, and optical deterioration may arise because of peeling of the Faraday rotator and polarizers.
The present invention has been made taking note of such problems, and what it aims is to provide a thermoset optical-purpose adhesive having a superior resistance to heat and moisture and a high transparency, and also to provide an optical isolator element and an optical isolator to both of which this adhesive has been applied.
Accordingly, the present inventors have made extensive studies upon the cause of deterioration in the conventional optical isolators.
As the result, it has turned out that the cause of the discoloration and deterioration which are due to temperature and humidity after curing is concerned with the compounding proportion of the amine type curing agent to the primary agent epoxy resin, and resides in that the amine type curing agent is compounded in a proportion of more than 45 parts by weight based on 100 parts by weight of the epoxy resin. Incidentally, as a solution therefor, a method may be contemplated in which the amine type curing agent is compounded in a small proportion, but this may cause another problem that the heat curing takes place insufficiently, and hence it has been difficult in practice to set its proportion to 45 parts by weight or less.
It has also turned out that the cause of the separation of the optical isolator element and the permanent magnets from the holder and that of the optical deterioration because of peeling of the Faraday rotator and polarizers in a thermo-hygrostat test are concerned with the combination of the optical element and the epoxy resin, the former being inorganic and the latter being organic. Incidentally, as a solution there for, it may be contemplated to carry out silane modification treatment in which a silane coupling agent is attached to the epoxy radical of the epoxy resin to improve its affinity for the inorganic optical element. However, in the epoxy resin, active-radical moieties such as a hydroxyl radical are also present in addition to the epoxy radical, and it is difficult to attach the silane coupling agent only to the epoxy radical. Hence, it has been difficult in practice to solve the problem.
After the present inventors have elucidated the causes in this way, they have continued further studies. As the result, they have discovered that the silane coupling agent can be made to react selectively on the epoxy radical of the epoxy resin when a metallic soap is made present together in carrying out the silane modification treatment, and that this also enables reduction of the compounding proportion of the curing agent to 45 parts by weight or less. The present invention has been accomplished on the basis of such a technical discovery.